Large quantities of ultrapure water or deionized (DI) water are consumed in the fabrication of IC devices, for instance, in a large number of wet cleaning and wet bench processes. The consumption of ultrapure water and DI water increases with the ever increasing wafer size. The DI water tank required for a 200 mm wafer wet bench is twice as large as one required for a 150 mm wafer wet bench. The consumption of water in a 200 mm wafer fab plant is therefore doubled from that of a 150 mm fab plant.
Many contaminants exist in a raw water supply. These include particles, organic materials, inorganic materials, microorganisms, bacteria and dissolved gases which include oxygen. When a raw water supply is fed through an ultrapure or DI water generating system, the contaminants are sequentially removed by a series of different types of filters, degassifiers, and ion-exchanger units.
When ultrapure water or DI water is used in the processing of IC wafers, one of the most critical impurities contained in water is the dissolved oxygen content. It is desirable that the dissolved oxygen in water to be kept as low as possible in order to prevent the growth of native oxide on bare silicon surfaces. This is normally achieved by making improvements in the performance of vacuum degassifiers. For instance, three popular degassification processes have been designed which include the hot water process that involves heating DI water to a temperature over 55.degree. C.; the nitrogen purging process which includes injecting a nitrogen flow into a DI water container; and a catalytic process which involves contacting DI water with a palladium compound contained in a vessel. It has been determined in semiconductor processing that it is generally desirable to keep the dissolved oxygen level under 100 ppb, and preferably under 50 ppb.
The ability to accurately monitor the content of dissolved oxygen in ultrapure water or DI water is therefore an important aspect of an IC wafer fabrication process. Presently, a commercially available dissolved oxygen analyzer is only capable of detecting the content of dissolved oxygen in water and performing a verification function, but not automatically calibrating itself. While it is not known, other than the growth of native oxide on bare silicon surfaces, the other detrimental effects of oxygen content in water, it is nevertheless agreed that the dissolved oxygen content in ultrapure water or DI water should be kept at a minimum, i.e., at a concentration of not higher than 50 ppb in order to minimize the potential detrimental effects.
Presently, since there is no standard calibration solution available for calibrating a dissolved oxygen analyzer, the analyzer is calibrated in atmosphere or by an air calibration method. A typical calibration curve is shown in FIG. 1. The air calibration method must be carried out manually. A sample flow is first shut off and an oxygen electrode is taken out of the flow cell and exposed to atmosphere until a stable reading in % saturation is obtained. For instance, the on-line reading is expressed by: EQU O.sub.2 [% sat]=cell current/calibration slope
While the calibration slope is calculated by the ratio of: EQU slope=cell current at 100% sat./100% sat value EQU O.sub.2 [ppb]=correction factor.times.O.sub.2 [% sat.]
In the conventional calibration method shown above, the reading is zeroed when no cell current is flown through the analyzer. However, this is not an absolute zero calibration. Moreover, when an oxygen electrode is exposed to a high concentration of oxygen, the silver anode discharges a large electrical current such that silver oxide film is readily formed on the anode surface which leads to a short lifetime of the silver electrode.
The conventional calibration method is sometimes supplemented by a Faraday verification procedure when the oxygen concentration of the sample is below 200 ppb. Periodically, a voltage is applied to the Faraday electrode to induce electrolysis in the sample solution and the formation of molecular oxygen and hydrogen. Based on Faraday's law and the measured flow rate, the current passing the electrodes can be adjusted automatically to produce an addition of approximately 20 ppb of oxygen. This is calibrated as a span point in a two-point calibration process. Since the oxygen electrode's response is perfectly linear to the oxygen concentration changes, the resulting electrode slope can be calculated with a single addition of oxygen.
As shown in FIG. 2, the on-line reading is represented by: EQU O.sub.2 [ppb]=cell current/calibration slope
While the verification slope can be expressed as: EQU slope=delta current/20 ppb
The Faraday verification does not provide a method for the calibration of absolute zero, instead, only provides a method of checking whether the slope is correct.
It is therefore an object of the present invention to provide a method for calibrating a dissolved oxygen analyzer that does not have the drawbacks or shortcomings of the conventional calibration methods.
It is another object of the present invention to provide a method for calibrating a dissolved oxygen analyzer that does not require the exposure of an oxygen electrode to the atmosphere and thus avoiding the oxidation of the silver electrode.
It is a further object of the present invention to provide a method for calibrating a dissolved oxygen analyzer that only requires the exposure of an oxygen electrode to a very low concentration of oxygen such that the lifetime of the silver electrode can be extended.
It is another further object of the present invention to provide a method for calibrating a dissolved oxygen analyzer that utilizes an oxidizer solution for the calibration of absolute zero.
It is still another object of the present invention to provide a method for calibrating a dissolved oxygen analyzer such that the lifetime of a silver anode utilized in an oxygen electrode can be at least doubled.
It is yet another object of the present invention to provide a dissolved oxygen analyzer which includes an oxidizer solution inlet and an oxidizer solution reservoir for determining an absolute zero point.
It is still another further object of the present invention to provide a dissolved oxygen analyzer which can be automatically calibrated by utilizing a saturated aqueous solution of sodium sulfite for determining the absolute zero point.
It is yet another further object of the present invention to provide a method for calibrating a dissolved oxygen analyzer by first setting a zero point on the analyzer by using an oxidizer solution and then setting a span point on the analyzer by filling a sample solution in the analyzer and flowing a cell current through a Faraday electrode until a 20 ppb oxygen is generated.